Abstract

When visualized as an operation on the Bloch sphere, the qubit "pi-over-eight" gate corresponds to one-eighth of a complete
rotation about the vertical axis. This simple gate often plays an important role in quantum information theory,
typically in situations for which Pauli and Clifford gates are insufficient. Most notably, if it supplements the set
of Clifford gates, then universal quantum computation can be achieved. The "pi-over-eight" gate is the simplest example of
an operation from the third level of the Clifford hierarchy (i.e., it maps Pauli operations to Clifford operations
under conjugation). Here we derive explicit expressions for all qudit (d-level, where d is prime) versions of this
gate and analyze the resulting group structure that is generated by these diagonal gates. This group structure
differs depending on whether the dimensionality of the qudit is two, three, or greater than three. We then discuss
the geometrical relationship of these gates (and associated states) with respect to Clifford gates and stabilizer
states. We present evidence that these gates are maximally robust to depolarizing and phase-damping noise, in
complete analogy with the qubit case. Motivated by this and other similarities, we conjecture that these gates
could be useful for the task of qudit magic-state distillation and, by extension, fault-tolerant quantum computing.
Very recently, independent work by Campbell
et al.
confirmed the correctness of this intuition, and we build
upon their work to characterize noise regimes for which noisy implementations of these gates can (or provably
cannot) supplement Clifford gates to enable universal quantum computation

Item Type:

Article

Additional Information:

This article is available at DOI:
10.1103/PhysRevA.86.022316 . We thank E. Campbell for helpful comments on a previous
version of this paper. M.H. was financially supported by
the Irish Research Council (IRC) as part of the Empower
Fellowship program. J.V. acknowledges support from Science
Foundation Ireland under the Principal Investigator Award No.
10/IN.1/I3013.